The present invention relates to a dressing tool for grinding wheels.
It has been known that dressing is an operation of removing the dull or loaded surface of the grinding wheel.
More particularly, the present invention relates to a dressing tool for grinding wheels which have a diamond coat on a base body and in which diamonds are held in a metallic bond in the coat. Such dressing tools may be cylindrical or profiled or alternatively wheels or dressing slabs.
The dressing operation is normally a mechanical shaping of a rotary grinding wheel, wherein the dressing tool is held against or applied to the working surface of the grinding wheel and producing controlled abrasion on the grinding wheel in such a fashion that the working surface of the grinding wheel will run perfectly true when rotating. A defined profile can be produced on the working surface of the grinding wheel.
The dressing operation is also used to produce a defined effective peak-to-valley height. When a workpiece is ground, the grinding wheel frequently tends to produce a defined roughness on the surface thereof. The degree of this roughness depends on the manner in which the dressing step on the grinding wheel was carried out. The effective peak-to-valley height is affected, on the one hand, by the kinematic dressing conditions, for example the rate of feed of the dressing tool on the grinding wheel surface in the direction of the axis of the grinding wheel. On the other hand, the grain size of the diamonds and the density of the diamond grain arrangement in the dressing tool also have a marked influence on the effective peak-to-valley height of the grinding wheel.
A dressing tool which is of simple construction but is versatile in use usually contains diamonds positioned in a systematic or random arrangement in a plane plate or so-called diamond coat. The diamond coat is joined to a base body which allows fixing to the grinding machine or to a device provided for dressing. Such a design of a dressing tool is termed a dressing slab.
The diamond coat is applied with its edge tangentially to the grinding wheel. Controlled abrasion on the grinding wheel is effected by diamond grains which are located in the region of the edge and are outwardly exposed to the grinding wheel.
In known dressing slabs, diamond grains are arranged in the plate in defined spacings. The diamond grains can lie as a single layer in one plane. Typical diamond grain sizes are between 0.5 mm and 1 mm. In cases where smaller diamond grains are used, they can also be arranged in several layers on top of one another.
During the dressings process of the grinding wheel, the grinding grains of which normally consist of corundum or silicon carbide, wear which occurs on the diamond grains of the dressing tool is relatively small. However, diamond grains must be held firmly by the surrounding metallic bonding material, so that they can adequately withstand the abrasive action of the grinding wheel. The bonding metal in which diamond grains are embedded must therefore also have a fairly high wear resistance. Typical bonding metals are alloys based on tungsten carbide and/or tungsten. If less wear-resistant bonding materials are used, such as, for example, cobalt, nickel or bronze, relatively rapid wear occurs on these metals, so that diamond grains embedded in the bond can break out of the bond at an unduly early stage. In the case of a dressing tool showing unduly rapid wear, however, the problem arises in maintaining precise dimensions during the dressing process, since the dimensions of the dressing tool may already change during the dressing process at predetermined feed rates. Moreover, the economic result of dressing would be unsatisfactory, because the dressing tool would wear out too rapidly, and unduly frequent replacement with a new tool would be necessary.
Diamond grains in the dressing tool are also subject to high thermal stresses due to intense friction on the grinding wheel. Diamond grades of high thermal stability are therefore chosen for such dressing tools. The disadvantage of the use of metal bonding based on tungsten or tungsten carbide resides in that relatively high sintering temperatures in the range of 900.degree. are necessary to produce this bond, so that diamond grains which are to be embedded in the bond suffer a greater or lesser amount of thermal damage on sintering. A process similar to the sintering of metal powder, and likewise conventional, is sintering in combination with impregnation with a liquid metal.
A production method in which the application of high temperatures is unnecessary comprises the use of a metal which can be electro-plated, such as, for example, cobalt, nickel, bronze or copper. However, these metals do not possess a very high abrasion resistance.
Recent studies have shown that the disadvantage of the lower abrasion resistance of these bonding materials which can be electro-plated is less serious if a dense arrangement of diamond grains in the diamond coat is provided. However, it was then found that the metal skeleton remaining between the diamond grains has relatively thin cross-sections and is therefore unable to hold the diamond grains in the best way. In fact, if diamond grains are merely enclosed by the metal in the metallic bonding, an adequately adhering joint between the enclosing metal and the diamond grains is not produced. This applies both to the abovementioned sintered metal bonds or impregnated metal bonds and to the metals which can be electro-plated.